You're right, NEC Article 250.50 is a good starting point, but it's not as specific as you might hope. Here's a breakdown of the requirements and where to find them in the NEC, along with explanations to address your specific concerns

NEC Requirements for Concrete-Encased Electrode (Rebar)

The relevant section is NEC 250.52(A)(3) - Concrete-Encased Electrode. This section details the requirements for a qualifying concrete-encased electrode, often called a "Ufer ground". The key takeaways are: (A)(3)(a) Rebar or Rod

The electrode must consist of at least 20 feet (6.0 m) of one or more bare, zinc-coated, or other electrically conductive coated or uncoated steel reinforcing bars or rods of not less than 1/2 in. (12.7 mm) in diameter, or consist of at least 20 ft (6.0 m) of bare copper conductor not smaller than 4 AWG.

(A)(3)(b) Encased in Concrete

The rebar or rod must be encased by at least 2 inches (50 mm) of concrete and located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth.

Addressing your questions


Rebar Size and Distance

The NEC does specify the minimum size of rebar:

1/2 inch in diameter. The spacing within the concrete is not directly specified for grounding purposes, however. The intent is to utilize the existing rebar structure within the concrete. You do not have to add 20 continuous feet of rebar if that rebar is not already there for construction. The concrete encasing is the crucial part.2. Grounding Points (Number of Connections)

The code does not specify how many points of connection are required. Only one connection to the electrode is required. However, the way in which the connection is made is important and should be accessible and protected from damage. See 250.64(A) for requirements on grounding electrode conductor sizing.

Important Considerations for your 9' x 44.5' Slab



Meeting the 20-Foot Requirement

The NEC requires at least 20 feet of rebar, 1/2" in diameter, encased in concrete. Your individual 4-foot rebar lengths are irrelevant as long as the total length of rebar that qualifies as a concrete-encased electrode adds up to at least 20 feet.

Continuous vs. Interconnected

The 20 feet does not need to be a single, continuous piece of rebar. It can be multiple pieces bonded together to create the required length. Typically, the rebar is already tied together by the installers. The NEC does not specifically define how this is done. But it needs to be able to handle faults as needed.

Bonding Rebar Together

If the rebar isn't naturally interconnected by ties, you'll need to ensure proper bonding between them. This is usually accomplished using approved clamps or welding. Welding is typically done in compliance with AWS D1.4.

Accessibility

The connection point to the rebar must be accessible (or made accessible with a removable cover). The connection has to be corrosion-resistant and mechanically protected.

When is it NOT Required? The code does not require a concrete-encased electrode. It is permitted as a grounding electrode. If you have other grounding electrodes (like a ground rod, ground plate, or metallic water pipe), you may not need to utilize the rebar. However, 250.50 requires you to bond all grounding electrodes together. Thus, if you have a qualifying concrete-encased electrode, you would need to bond to it. 250.104 Bonding of Piping Systems and Exposed Structural Metal. This part is relevant if your rebar is considered "exposed structural metal" and is likely to become energized. This is important to understand as it is not the same as grounding but is closely related.

In Summary


Minimum Rebar Size

1/2 inch diameter.2.

Minimum Total Length

20 feet of rebar in direct contact with the Earth.3.

Concrete Encasement

At least 2 inches of concrete surrounding the rebar.4.

Continuity

The 20 feet can be made up of multiple pieces if properly bonded together.5.

Single Connection Point

Only one connection to the electrode is required.6.

Accessibility

The connection must be accessible.

Recommendations



Consult with a Licensed Electrician

Grounding is a critical safety issue, and the NEC can be complex. An electrician can assess your specific situation, ensure code compliance, and make the connection properly.

Check Local Amendments

Your local jurisdiction may have amended the NEC with more specific requirements for grounding rebar. Always check with your local building department.

Document Everything

Keep records of the grounding system, including photos of the connections before the concrete is poured, and any inspection reports.

Ground Rods in addition to Ground Wire

I would recommend using ground rods in addition to using a ground wire connected to your concrete-encased electrode.By following these guidelines and consulting with a qualified professional, you can ensure a safe and code-compliant grounding system for your slab.
Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional electrical advice. Always consult with a licensed electrician for any electrical work. Flag for review